and mechanical properties,and others such as opacity, conductivity and permeability through the final material. Silica has good mechanical properties, thermal stability, and the homogeneous combination of organic and inorganic fractions in a single phase gives unique possibilities. The modification of the surface of siliceous materials by covalent binding with organic species, in particular organosilanes, is being studied for the design of hybrid materials with properties directed towards advanced technological applications, such as hydrophobicity.